Types of Reactors

Commercial Reactors Around the World

There are six main reactor types in use around the world. The various designs use different concentrations of uranium for fuel, different moderators to slow down the fission process, and different coolants to transfer heat.

The most common reactor type is the pressurized water reactor (PWR), representing 250 of the world’s 392 reactors now operating.

Commercial Reactor Types Around the World in 2014

Reactor Type

Fuel

Moderator

Coolant

Number

Pressurized water reactor (PWR)

Enriched UO₂

Water

Water

250

Boiling water reactor (BWR)

Enriched UO₂

Water

Water

58

Pressurized heavy water reactor “CANDU” (PHWR)

Natural UO₂

Heavy water

Heavy water

48

Gas-cooled reactor (GCR)

Natural U (metal), enriched UO₂

Graphite

Carbon dioxide

16

Light water graphite reactor (LWGR)

Enriched UO₂

Graphite

Water

15

Fast breeder reactor (FBR)

PuO₂ and UO₂

None

Liquid sodium

2

Pressurized Water Reactors

Pressurized water reactor.

Pressurized water reactors (PWRs) are the most common type of reactor worldwide. PWRs use ordinary (or “light”) water as both coolant and moderator. The coolant is pressurized to stop it from flashing into steam keep it liquid during operation. Powerful pumps circulate the water through pipes, transferring heat that boils water in a separate, secondary loop. The resulting steam drives the electricity-producing turbine generators.

Boiling water reactors (BWRs) make up 15% of reactors globally. In a BWR, light water acts as both coolant and moderator. The coolant is kept at a lower pressure than in a PWR, allowing it to boil. The steam is passed directly to the turbine generators to produce electricity. While the absence of a steam generator simplifies the design, radioactivity can contaminate the turbine.

Also known as CANDU reactors, pressurized heavy water reactors (PHWRs) represent about 12% of the reactors in the world and are used at all Canadian nuclear power generation stations. They use heavy water as both coolant and moderator, and use natural uranium as fuel. As in a PWR, the coolant is used to boil ordinary water in a separate loop. CANDU reactors can be refuelled without shutting the reaction down.

Gas-cooled reactors (GCRs) are in use only in the United Kingdom. There are two types, the Magnox (named from the magnesium alloy used to clad the fuel elements) and the advanced gas-cooled reactor (AGR). Both types use carbon dioxide as the coolant and graphite as the moderator. The Magnox uses natural uranium as fuel, while the AGR uses enriched uranium. Like CANDU reactors, these designs can be refueled while operating.

Light Water Graphite Reactors

Light water graphite reactors (LWGRs) are used in Russia, with ordinary water as the coolant and graphite as the moderator. As with BWRs, the coolant boils as it passes through the reactor and the resulting steam is passed directly to turbine generators. Early LWGR designs were often built and operated without the safety characteristics and features required elsewhere. The well known 1986 accident at Chernobyl (Ukraine) happened to a reactor of this type.

Because slow neutrons are more likely to split uranium atoms, most reactor types are designed to make use of them. In contrast, fast breeder reactors (FBRs) use fast neutrons to convert materials such as uranium-238 and thorium-232 into fissile materials, which then fuel the reactor. This process, combined with recycling, has the potential to increase available nuclear fuel resources in the very long term. FBRs operate mainly in Russia.

The modern small modular reactor (SMR) is designed to be built economically in factory-like conditions (rather than onsite), and with capacities between approximately 10 MWe and 300 MWe.

Small Modular Reactors

Small modular reactor.

There is growing interest in SMRs to provide electricity to service small electricity grids, and possibly to provide heat for resource industries. SMRs can also be added incrementally to larger grids as demand grows. The IAEA estimates that as many as 96 SMRs could be operational worldwide by 2030.

Some SMR designs are in advanced stages of development, including several designed to be fully underground, minimizing land use, staffing, and security needs. Some designs include passive safety systems, and can operate for up to four years without refuelling.

Reactor Lifetimes

Some first-generation reactors, such as the Magnox gas-cooled reactors in the United Kingdom, are still in service, though they are approaching the end of their operational lives. Many of today’s reactors were built in the 1970s and 1980s and will approach lifetimes of 40 years beginning around 2015. However, studies based on operating and materials experience have revealed no major technological issues inhibiting longer operational lives for many reactors, particularly PWRs and BWRs.

Careful monitoring of plant performance, analysis of operating experience, modernization programmes and refurbishments offer good prospects for life extensions at many plants. For example, as of 2014, ten CANDU reactors are scheduled for refurbishment to extend their operational life by 25 to 30 years and beyond. Other countries such as the United States and the Russian Federation are also planning to extend the lifetimes of existing reactors. In many countries decisions on plant lifetimes are made through the periodic renewal of operating licences, which involve comprehensive safety analyses using the latest methods, information and safety requirements.

Next generation reactors will be designed to operate for longer periods up to 60 years.